Oscillators are used in various circuits, integrated circuits (chips), and systems. Oscillators often include an inductor, a capacitor, a negative resistance element to maintain oscillation and a variable capacitor or varactor for controlling the frequency of the oscillator. An oscillator controlled with a digital control word is referred to as a digitally controlled oscillator (DCO). An oscillator controlled with an analog control voltage is referred to as a voltage controlled oscillator (VCO).